Field of the Invention
This invention relates to an ignition apparatus used mainly for operations of an internal combustion engine.
Description of the Related Art
In recent years, the problems of environmental conservation and fuel depletion have been raised. In the automobile industry, it is an urgent task to deal with these problems. An example of the solution is a method of remarkably improving fuel consumption by engine downsizing utilizing a supercharger or by weight reduction.
It has been known that when an internal combustion engine (hereinafter also referred to as an “engine”) is in a highly supercharged state, the pressure in the combustion chamber thereof becomes very high even in a state that does not accompany combustion, so it becomes difficult to generate a spark discharge for starting combustion. One of the reasons is that the voltage required for causing dielectric breakdown between the high voltage electrode and the GND electrode (i.e., in the gap) of the spark plug becomes very high and exceeds the withstanding voltage value of the insulator portion of the spark plug.
In order to resolve this problem, research has been conducted to raising the withstanding voltage of the insulator portion. In reality, however, it is currently difficult to ensure sufficient withstanding voltage for meeting the demand, so the circumstance is that there is no other choice than choosing the means to narrow the gap distance of the spark plug. Nevertheless, when the gap of the spark plug is narrowed, another problem arises that the quenching effect of the electrode portion tends to bring about undesirable effects, causing degradation in startability and degradation in combustion performance.
In order to solve this problem, it appears possible to take an avoiding means of imparting an energy that exceeds the quenching effect, that is, the thermal energy taken away by the electrode portion, by way of spark discharge, or causing combustion at a location as far as possible from the electrode. Accordingly, an ignition apparatus disclosed in, for example, Patent Document 1 has conventionally been proposed.
The ignition apparatus disclosed in Patent Document 1 is such that it generates a spark discharge in a spark plug gap with a conventional ignition coil and causes a high frequency current to flow into the path of the spark discharge through a diode and a mixer, whereby it makes possible to form high energy spark discharge and discharge plasma that expands over a wider range than normal spark discharge.    Patent Document 1: JP-A-2011-099410
The conventional ignition apparatus disclosed in Patent Document 1 above contains a diode with a high withstanding voltage. Currently, the high withstanding voltage diode is manufactured with a stack structure using a lead solder, so its size has been made small. However, it has been difficult to adopt such a structure because of the viewpoint of lead-free design in recent years. When the lead-free design is employed, it becomes necessary to ensure a sufficient physical insulation distance. Consequently, problems arise that size reduction becomes difficult and the manufacturing cost becomes very high. On the other hand, it appears possible to pass a high-frequency alternating current of 10 MHz or higher between the electrodes of the spark plug. However, a device that is capable of high-frequency and high current switching and also ensuring reliability is very costly, so this also causes the problem of very high manufacturing cost.